A、It was rising. B、It was lowering. C、It remained unchanged. D、It was fluctuating. B细节题。通过浏览选项可知，本题问的是变化趋势。是上升?下降?未变?还是波动?文中关键句子

Recently, over 30 million Americans of both sexes and all ages have shown great interest in walking. They have a strong desire to keep fit. By nature, Americans are enthusiastic and energetic about hobbies or pastimes. As a result, there are walkers’ clubs to join in and many books and magazines to read about walking.
    Walking is probably the most overlooked way to physical fitness. Like other exercises, it can help control weight, strengthen the heart and lungs, ease stress and build up energy. Unlike many exercises, walking is simple, easy and cheap.
    Middle-aged men suffer from heart disease, the nation’s number one killer. Thus, they are strongly interested in walking. In fact, many doctors encourage their patients to become more physically active. It is interesting to note that the rate of heart disease began to decrease between 1992 and 1994 and it is still decreasing.
    Remember that it is possible to overdo anything, even walking. Many confused walkers overdo and finally suffer from ankle and foot damage. Be careful not to overexert yourself, especially in hot weather.
29. Who likes the physical exercise of walking in the United States?
30.Why are Americans so interested in walking?
31.What is true about the rate of" heart disease between 1992 and 1994 in the United States?